Devices implanted in the body for electrical cardioversion or pacing of the heart are well known. More specifically, devices implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm.
Cardiac pacing units are typically comprised of a signal generating unit called a pulse generator and an elongated lead that passes through a vein and into the heart. One end of the lead is attached to the surface of the heart and the other end of the lead is attached to a connection block just outside the pulse generator. An electrode is in the portion of the lead attached to the heart. The electrodes can be used to deliver signals to the heart to defibrillate the heart if its beating too fast or to pace the heart if it is not beating fast enough. The electrodes can also be used to sense certain electrical signals made by the heart to trigger a heart beat. The pulse generator monitors the signals made by the heart and uses this as an input to determine the type of signal needed to be delivered to the heart to correct for an arrhythmia or irregular heart beat.
The lead portion of the cardiac pacing unit is quite long. The lead contains an electrical conductor used to transfer signals from the heart to the pulse generator, and to transfer signals from the pulse generator to the electrodes attached to the heart. The long lead may act like an antennae and is susceptible to stray, external electromagnetic fields which causes noise within the long lead generally called electromagnetic interference ("EMI").
From time to time, humans are subject to external fields which could become problematic to the pulse generator. These fields are broadly known as electromagnetic fields and cause what is broadly known as electromagnetic interference (EMI). This EMI is source generated. Some typical examples of these EMI sources include cellular telephones, anti-theft devices used in department stores, toy cars which are radio controlled, radios and others. In addition, much EMI is generated by sources such as television broadcast antenna, radio stations, satellites and the like. EMI is even caused by electrical current within a typical household.
In certain situations, this EMI can become problematic for the pulse generator. These problems are typically manifest as the device sensing these noise signals as heart signals. When this occurs, the device makes inappropriate therapy delivery decisions and can cause patient harm. More specifically, in the presence of electromagnetic interference or EMI, a signal or signals may be created in a long wire such as the long lead used in a pacing system. The electronics housed within the signal generator of the pacing system generally recognize stray signals or EMI as noise. The possibility exists that EMI produced in the lead and created by an electromagnetic field, may replicate or be close to a signal from the heart. The electronics in the signal generator of the pacing system could misinterpret the EMI as a real signal. If this happens, the result could be disastrous. For example, the signal generator may send a large signal to defibrillate the heart when in fact no such signal is needed.
With more EMI generators being produced every day, it has become increasingly important to filter out this unwanted noise, before the noise gets to device electronics.
The current approach shows the EMI filter capacitor which is located internal to the device and acts as a capacitive filter to high-frequency EMI noise. U.S. Pat. Nos. 4,152,540; 4,424,551; and 5,333,095 are such typical applications of a feedthrough filter. This approach is generally effective but has several drawbacks. One of the drawbacks is that the EMI noise still gets inside the pulse generator. Even though the filter is the first thing the unwanted EMI encounters, it still enters the pulse generator. Depending on the frequency content of the EMI, a portion, or all of the noise could reflect off of the lead prior to being filtered. This renders the capacitor somewhat useless to certain types of EMI. Since new devices are coming on the market daily, it is difficult to anticipate every possible EMI situation.
Another drawback is that allowing space inside the pulse generator for this filter has become inefficient and cumbersome from a mechanical design point of view. This is due to the necessity of allowing a certain amount of headroom inside the device for the capacitive element. In addition, the capacitor is a fairly fragile component such that care must be taken not to damage the component during pulse generator assembly or during the manufacturing process.
During manufacture, a laser weld is used to hermetically close the housing of the pulse generator. The capacitor gets very hot during laser welding and has been known to crack during this process. Such a defect cannot be found until late in the manufacturing process during final electrical testing. Such a crack ruins the entire pulse generator since the outputs short together. Since the pulse generator is a difficult component to inspect, build and control during assembly, it is very expensive to assemble. If such a defect cannot be found until final inspection, the expense associated with assembly is lost.
It is also important to conserve space inside the pulse generator of the pacing system so that a larger battery can be used or a smaller device can be made. Larger batteries generally last longer than smaller batteries. This is important to the person that uses the a pacing system. The pulse generator and the lead is typically implanted in the body of the user. The pulse generator is placed below the skin. When the battery wears out, the batteries are typically "replaced" by replacing the pulse generator. Thus, the longer the battery lasts, the longer the time between surgeries to replace the pulse generator.